JPH079453B2 - Quantum magnetic flux parametron signal detection method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconducting circuit that operates at cryogenic temperatures, and more particularly to a signal detection method for a quantum flux parametron, which is a parametron type superconducting switching circuit using Josephson elements. Regarding

(Prior Art) Quantum Flux Param, which is a parametron-type switching circuit that uses a superconducting element that exhibits the Josephson effect (hereinafter referred to as the Josephson element).
etron: hereinafter abbreviated as QFP) is known in the technical field, and the 1959 RIKEN Symposium Proceedings 1-
3 and 48-78, 1994 RIKEN Symposium Proceedings Proceedings 1-13, 1986 RIKEN Symposium Proceedings Proceedings 53-58. QFP is a switching circuit based on a new operating principle that uses quantized DC magnetic flux as a signal medium, and is excellent as a computer element because it operates at high speed with extremely small power consumption.
Also, since it weakly signals are amplified with high gain, it has excellent properties as an analog circuit such as a magnetic sensor.

Since QFP operates with extremely low energy, it can achieve high sensitivity, but on the other hand, it is more susceptible to the surrounding measurement environment and noise. In the conventional technology, this QFP
In order to detect the output current signal of the above as a voltage signal, the method of utilizing the latching characteristic of the Josephson element from the superconducting state to the voltage state has been performed. Japanese Patent Application No. 60-241469 shows an example of this detection method.
The method disclosed therein uses a detection circuit in which a first and a second control line are arranged adjacent to each other in a superconducting loop including two Josephson elements. A bias current of a certain value that does not put the two Josephson devices into a voltage state by itself is applied to this detection circuit. In this state, the output current of the QFP is passed through the first control line and the second control line is fed. Sweep current is applied. The sweep current value when the two Josephson devices transition from the superconducting state to the voltage state is sampled and held, and the signal output of the QFP is detected. In this type of detection circuit, it is necessary to temporarily cut (zero) the bias current in order to initialize the circuit (to the superconducting state) after the voltage circuit once transits to the voltage state.

(Problems to be Solved by the Invention) In the conventional method, before the QPF is excited,
It is necessary to transiently transit the bias current supplied to the signal detection circuit from zero to a finite value. The induced noise generated by the transition of the bias current and the bias current which is not stable immediately after the transition adversely affect the QFP in the sensitive state at the start of the excitation and cause a malfunction. However, if the bias current is changed sufficiently before the start of excitation, there is no fear of malfunction, but this method causes another problem that high-speed signal detection cannot be performed.

(Means for solving the problem) Signal detection of the quantum magnetic flux parametron is performed by SQUID (Supercond
ucting Quantum Interfeence Device) is a signal detection method of the quantum magnetic flux parametron, when the quantum magnetic flux parametron is in a non-excited state, the bias current to the SQUID does not transit from zero to a finite value, specifically, , (1) The bias current is always supplied to the SQUID, or (2) the supply of the bias current to the SQUID is started after the quantum flux parametron reaches an excited state, and in the case of (1), the quantum flux parametron is The first output signal value of the SQUID in the non-excited state is measured, and the SQ when the quantum flux parametron is in the excited state immediately after the non-excited state
The second output signal value of the UID is measured, the difference between the first output signal value and the second output signal value is obtained, and the signal of the quantum magnetic flux parametron is detected by this difference. In the case of (2), When the quantum flux parametron is in the excited state and the bias current to the SQUID is supplied, the first output signal value of the SQUID is measured, and the quantum flux parametron is in the non-excited state adjacent to the excited state. The second output signal value of the SQUID is measured when the bias current to the SQUID is still supplied, and the difference between the first output signal value and the second output signal value is obtained. The above-mentioned problems can be solved by performing the signal detection of the magnetic flux parametron.

SQUID is a superconducting device that is already used as a means for measuring weak magnetic flux or current at cryogenic temperature. Various technologies using this SQUID are described in, for example, the Institute of Electrical Engineers, "Josephson Effect 《Basic and Application》. , May 1978, disclosed in detail to Corona. Conventionally, SQUID is used to measure extremely weak magnetic flux, which is about 1/1000 of quantum magnetic flux (2.07 × 10 ^-15 Wb). SQUID includes DC-SQUID driven by DC power supply and rf-SQU driven by AC power supply.
There are two types of ID.

(Operation) A DC bias current that makes this SQUID a voltage state is applied to the dc-SQUID by itself, and unlike the superconducting-voltage state transition as in the conventional method, a finite voltage value is output from the QFP. Signal detection is performed by changing the current signal.

The rf-SQUID is driven by an AC power supply, and the voltage amplitude generated in the LC resonant circuit that is transformer-coupled to the rf-SQUID is
Signal detection is performed by changing with the current signal from.

The rf-SQUID is driven by an AC voltage. Generally, the drive frequency is higher than the frequency range of the input signal by, for example, about two orders of magnitude, and since it is a sine wave with a single frequency spectrum, the noise caused by this drive power source is It can be easily removed with a filter.

Both dc-SQUID and rf-SQUID used in the present invention do not use the latching characteristic from the superconducting state to the voltage state, and the bias current is transiently changed from zero to a finite value before the signal from the QFP is applied. There is no need for transitions, thus eliminating the cause of QFP malfunction.

The measurement resolution of SQUID is closely related to the measurement frequency range, and there is a drawback that the measurement resolution cannot be increased when the measurement frequency range is widened. On the other hand, the QFP is an element that amplifies a weak input magnetic flux and outputs a relatively large magnetic flux of about 1/4 of the quantum magnetic flux. Therefore, SQUID does not need high measurement resolution for QFP signal detection and can measure a wide frequency range. In this sense, SQUID is suitable for QFP measurement.

In the present invention, the current signal of the QFP is detected by the change in the finite voltage value generated by the SQUID, and the output voltage of the SQUID includes the offset. It was difficult for the conventional SQUID technology to correct this offset signal and extract the true signal. In the present invention, QFP
This offset signal can be easily removed because it is combined with. That is, when measuring the output signal of the QFP, the output signal of the SQUID when the QFP is not excited is used as a reference, and the output signal when the QFP is excited is measured. .

Note that the technology disclosed heretofore does not disclose the technology in which SQUID is applied to QFP.

(Examples) Hereinafter, the present invention will be described using examples. FIG. 1 is a first embodiment of a QFP output detection method according to the present invention. QFP
Consists of a superconducting loop 5 formed by connecting a circuit pair consisting of a first Josephson element 1, a first excitation inductor 3 and a second Josephson element 2, a second excitation inductor 4. An input line 7 and load inductors 8 and 8'are connected to the superconducting loop 5. The current flowing in the load inductor is detected, but if the detection circuit and load inductor are strongly coupled, the effect of the detection circuit will appear in QFP. Therefore, the load inductor is divided for the purpose of adjusting the coupling, and the detection circuit is coupled to a part 8'of the load inductor. An excitation line 6 is arranged near the superconducting loop 5, and the excitation line 6 is magnetically coupled to the first and second excitation inductors 3 and 4. An excitation current is supplied to the excitation line 6 from an excitation current source 9. The circuit operation of the QFP will be described below. For simplicity, circuit operation will be described using current. First, a weak input current (input magnetic flux) is directly injected into the superconducting loop 5 via the input line 7 without applying the excitation current. After that, when an exciting current is applied to the exciting wire 6, an output current amplified corresponding to the direction of the input current flows through the load inductor 8. To measure this output current, dc-SQUID
use. The dc-SQUID10 consists of two Josephson elements 11 and 12
The inductors 15 and 16 and the inductors 13 and 14 magnetically coupled to the load inductor 8'of the QFP form a superconducting loop. The dc-SQUID10 the bias current I _B from the current source 17 is supplied. The voltage of dc-SQUID is output to the outside via the output wiring 18. Next, the current detection circuit dc-
The operation of SQUID will be described. Magnetic flux detection methods using dc-SQUID are known in the prior art, for example, J. Clark “Advan.
cesin SQUID Magnetometers "IEEE Trans.Electron Devi
ce Vol. ED-27, No. 10, pp. 1896-1908, Oct. 1980. The Josephson element of the dc-SQUID10 is a low junction capacitance element, or has a structure in which a resistor is connected in parallel to the junction and has a McCumber constant smaller than 1. There is no hysteresis in the voltage-current characteristics of the Josephson junction at this time. Figure 2 shows the voltage-current characteristics of a dc-SQUID using this device. That is, the voltage / current characteristics have no hysteresis, and the maximum superconducting current (critical current value) changes due to the magnetic flux interlinking with this dc-SQUID. For example, second
In the figure, the characteristic (1) is obtained when the interlinking magnetic flux is zero, and the characteristic (2) is obtained when it is not zero. When the bias current is I _B , the output voltage of the dc-SQUID is V ₁ and V ₂ from FIG. 2, respectively. Therefore, it is clear that the output current of the QFP can be detected by observing the output voltage of the dc-SQUID. In this configuration, since only the DC power supply is used to drive the dc-SQUID, the induced noise from the power supply for driving the detection circuit is extremely small. Further, since the dc-SQUID is driven by a DC power supply, the detection operation can be speeded up.

The DC offset is included in the output signal of the dc-SQUID. This is because, from the operating principle of dc-SQUID, the critical current of dc-SQUID does not become zero when the input signal is zero.
Conventionally, in order to detect this offset signal, an operation of interrupting the input signal to create a state where the input signal is zero and measuring the output signal at that time has been required. Further, since the detected offset signal is an extremely small amount of signal, it is easily affected by an external circuit, and this appears as a phenomenon in which drift is large in measurement. With the SQUID technology disclosed in the past, correction of DC offset was extremely troublesome. In the embodiment of the present invention shown in FIG. 1, the correction of the DC offset of the SQUID uses the excitation operation of the QFP. FIG. 3 shows a time chart according to the first method when the QFP signal is measured by dc-SQUID. The QFP is shown at B and is excited by the excitation signal. In response to the excitation signal, QFP outputs A or A'signal. Where A is the positive input signal,
A'is when the input signal is negative. SQUID outputs signals D and D'in response to the output signal of this QFP. Here, when QFP is not excited, for example, at time Ta, QFP
, The input signal of the dc-SQUID is zero. Therefore, the output of dc-SQUID at this time Ta is a DC offset signal. When the QFP is excited, the dc-SQUID corresponding to the output signal of the QFP outputs the signals D and D '. Here, it is assumed that D is a positive input signal and D'is a negative input signal. The output signal when QFP is excited is, for example, time Tb based on the DC offset of dc-SQUID.
Then, f and f'can be measured. As described above, in the embodiment of FIG. 1, the DC offset of the dc-SQUID is calibrated when the QFP is not excited, and the output signal of the QFP is dc based on that value.
-Measure with SQUID. Therefore, since the DC offset of the dc-SQUID, which has conventionally been troublesome, is calibrated for each measurement, it is possible to prevent a decrease in measurement accuracy due to drift and to avoid the DC offset calibration that conventionally requires a complicated operation. A circuit for implementing this DC offset removal method is shown in FIG. 8 and its operation will be described later.

In general, the circuit configuration of FIG. 1, the magnetic flux generated by the bias current I _B flowing through the dc-SQUID affects the QFP. In this case, if the magnetic fluxes generated by the inductors 13 and 14 of the dc-SQUID 10 are set to be the same, the magnetic fluxes generated by the inductors cancel each other out, and as a result, the influence of the dc-SQUID does not affect the operation of the QFP. For example, if the structure of the dc-SQUID 10 is made symmetrical, the critical currents of the Josephson elements 11 and 12 are made the same, and the values of the inductors 13 and 14 and the values of the inductors 15 and 16 are made the same, the target circuit can be constructed. it is obvious.

FIG. 4 shows an auxiliary method for further reducing the influence of dc-SQUID on QFP in the circuit configuration of FIG. In this circuit configuration, a method of applying a correction magnetic flux via the correction inductor 21 and the correction current source 22 which are magnetically coupled to the load inductor 8 is used.

FIG. 5 shows another auxiliary method for reducing the influence of dc-SQUID on QFP in the circuit configuration of FIG. In this method, the correction current from the correction current source 22 is directly injected into the QFP.

Since the power supply current of the dc-SQUID is DC in FIGS. 4 and 5, the correction current may be DC in both FIGS. 4 and 5, and therefore the correction can be performed with high accuracy.

FIG. 6 is a time chart of signals in the second method for reducing the influence of dc-SQUID in the circuit configuration of FIG.
The excitation state of QFP depends on the input signal at the initial stage of excitation. Therefore, if the bias current of the dc-SQUID 10 is cut off at the time of starting the excitation of the QFP, the effect of the dc-SQUID on the QFP can be practically eliminated. In the time chart of FIG. 6, QFP is excited by the excitation signal (current) shown in B from time T1 to T3. QFP corresponds to the excitation signal
Alternatively, the signal A'is output. Here, A is the case where the input signal is positive and A'is the case where the input signal is negative. The dc-SQUID for detecting the signal is biased with the pulse current shown at C. That is, the bias current of the dc-SQUID 10 is a pulse current that is zero at the excitation start time T1 of the QFP and holds a constant value from the time T2 to the time T4. At this time, the dc-SQUID 10 outputs signals D and D'according to the output signal of the quantum magnetic flux parametron. The output signal of this dc-SQUID is from T2 to T
Up to 3, the QFP is in the excited state, so D and D'depending on the output signal. In addition, since QFP is in the non-excited state from time T3 to T4, the output signal of dc-SQUID becomes a DC offset. Therefore, the output signal of QFP may be the difference between the output signals of time Tb and time Tc of dc-SQUID.

FIG. 7 shows a second embodiment according to the present invention. In this embodiment, a part of the load inductor 8'of the QFP and the dc-SQUID inductor are made common, and the QFP and the dc-SQUID are directly connected. Obviously, with this structure, the measurement shown in the time chart shown in FIG. 3 or FIG. 6 can be performed.

FIG. 8 is a structural example of an external circuit for operating the signal detection circuit according to the present invention. The output wiring 18 of the combined circuit 101 of QFP and SQUID as shown in FIGS. 1, 4, and 5 is connected to the input side of the amplifier 102. The output of the amplifier 102 is connected to the data input terminal IN of the sample and hold circuit 104 and one data input terminal of the arithmetic circuit 105. The data output of the sample and hold circuit 104 is connected to the other input of the arithmetic circuit 105. Of combination circuit 101
The QFP is excited by the power supply 100, but the signal of the power supply 100 is
Input to the trigger signal generator 103, and the trigger signal generator 103
Generates a trigger signal at the time of non-excitation, for example, at time Ta in FIG. When the trigger signal is input, the sample-and-hold circuit 104 newly captures the data of the amplifier 102, stores the value, and outputs it. The arithmetic circuit 105 outputs the difference between the two input signals to the terminal 106. With the configuration of FIG. 8, the sample-and-hold circuit 104 captures and stores the signal of dc-SQUID when the QFP is not excited as a DC offset signal, and outputs it as the attenuation of the arithmetic circuit 105. When the QFP is excited, a signal with the DC offset removed is output to the terminal 106 as shown in FIG. Here, since the trigger signal is periodically generated when the QFP is not excited, the output signal (hold signal) of the sample and hold circuit 104 is updated in synchronization with the excitation signal.

In the above examples, the explanation was made using dc-SQUID,
It is clear that the same function can be realized by using rf-SQUID. FIG. 9 shows an embodiment of the present invention using rf-SQUID. In FIG. 9, a superconducting loop 50 including a Josephson element 51 and inductors 52 and 53 is coupled to the load inductance 8 ′ of the QFP via the inductor 52.
Furthermore, a resonance circuit 55 including an inductor 54 and an electrostatic capacitance 55 is coupled to the superconducting loop 50 via the inductors 53 and 54. The resonant circuit 57 is driven by a high frequency power source 57, and the signal amplitude of the resonant circuit is observed via terminals 58 and 59. rf
-The operation of SQUID is that the output signal amplitude of the resonance circuit 56 changes due to the current flowing in the load inductor of the QFP.
If the signal amplitude is rectified and observed, the operation of the entire circuit is similar to that of the dc-SQUID.

(Effects of the Invention) As described above, by using the present invention, it is possible to provide a quantum flux parametron detection circuit that detects a wide-band signal with low noise without affecting the quantum flux parametron. Therefore, according to the present invention, it is possible to realize a high-speed and highly accurate magnetic sensor, for example.

[Brief description of drawings]

FIG. 1 is an embodiment of the present invention and is a circuit diagram when magnetically coupling a dc-SQUID, and FIG. 2 is a graph showing the relationship between the bias current value of the dc-SQUID and the generated output voltage. , FIG. 3 is a diagram showing a time chart in the first method when measuring the signal of the quantum magnetic flux parametron with dc-SQUID, and FIG. 4 is a circuit diagram of FIG. Circuit diagram that adopts an auxiliary method to further reduce the influence, Fig. 5 is a circuit diagram that employs another auxiliary method to reduce the influence of dc-SQUID on QFP in the circuit configuration of Fig. 1. , FIG. 6 is a diagram showing a time chart of signals in the second method for reducing the influence of dc-SQUID in the circuit configuration of FIG. 1, and FIG. 7 is another embodiment of the present invention. -Circuit diagram when SQUID is directly connected, Fig. 8 shows offset generated by using SQUID FIG. 9 is a circuit diagram showing a configuration example of an external circuit for removing a voltage, and FIG. 9 is a circuit diagram of an embodiment of the present invention using rf-SQUID. 1 ... 1st Josephson element, 2 ... 2nd Josephson element, 3 ... 1st excitation inductor, 4 ... 2nd excitation inductor, 5 ... Superconducting loop, 6 ... Excitation line, 7 ... Input line, 8, 8 '... Load inductor, 9 ... Excitation current source, 10 ... dc-SQUID, 11, 12 ... Schosefson element, 13, 14, 15, 16 ... Inductor, 17 ... ... current source, 18 ... output wiring, 100 ... power supply, 101 ... with the QFP shown in FIG. 1, FIG. 4 or FIG.
Circuit diagram of a circuit that is combined with SQUID, 102 ... Amplifier, 103 ... Trigger signal generator, 104 ... Sample and hold circuit, 105 ... Arithmetic circuit, 106 ... Terminal

Claims (2)

[Claims] 1. At least one Josephson for directly or magnetically coupling the signal detection of a quantum flux parametron having a structure in which a load inductor is connected to a superconducting loop including two Josephson elements and an excitation inductor. SQUI consisting of a second superconducting loop containing elements
A method of detecting a signal of a quantum magnetic flux parametron performed by D, wherein a bias current is always supplied to the SQUID, and the quantum magnetic flux parametron is in a non-excited state.
Measuring a first output signal value of SQUID, measuring a second output signal value of the SQUID when the quantum flux parametron is in an excited state immediately after the non-excited state, A method of detecting a signal of the quantum magnetic flux parametron, wherein a signal from the quantum magnetic flux parametron is detected based on a difference from the second output signal. 2. Quantum flux parametron signal detection of a structure in which a load inductor is connected to a superconducting loop including two Josephson elements and an excitation inductor, and at least one Josephson directly or magnetically coupled to the load inductor. SQUI consisting of a second superconducting loop containing elements
A method for detecting a signal of a quantum magnetic flux parametron performed by D, wherein after the quantum magnetic flux parametron reaches an excited state, supply of a bias current to the SQUID is started, and the quantum magnetic flux parametron is in the excited state,
The SQUID when bias current is being supplied to the SQUID
The second output signal of the SQUID when the quantum flux parametron is in a non-excited state adjacent to the excited state and the bias current to the SQUID is still being supplied. Is measured, the difference between the first output signal and the second output signal is obtained, and the signal detection of the quantum magnetic flux parametron is performed based on this difference.